Presently, CT scan is widely used in the field of medicine. Different tissue structures in a CT image can be discriminated by the different attenuations of x-rays with different energies in a material.
At present, generally there are two methods for collecting dual energy data. In one method, dual energy is achieved by rapidly switching tube voltages from projection to projection, thereby obtaining data for distinctive energies in an alternate way in respective projection directions during one scan. In the other method, a dual-source technology is applied, in which each of the tubes emits rays with a distinctive energy to obtain data for two different energies during one scan.
For one material, such as water or bone, x-ray beams with two different energies have different attenuations, and thus the pixel values of the obtained CT images are also different, by which two components can be discriminated in the image. For example, a CT image reconstruction may be performed from data obtained in a scan conducted at two different energies, resulting in two images I1 and I2. The proportions of water and bone at each point of the image may be simply calculated from the following two equations:Water=Coeff1*I1+Coeff2*I2Bone=Coeff3*I1+Coeff4*I2If the combination coefficients Coeff1, Coeff2, Coeff3, Coeff4 are known, the proportions of water and bone can be easily calculated according to the images I1 and I2 respectively corresponding to two different beam energies. Generally, those coefficients are obtained by scanning a simple object for correction in advance and applying certain correction algorithms.
However, the process of obtaining the dual-energy data has a high hardware requirement. For example, in one of the two methods for collecting data described above, it is required to switch the tube voltages among different projections quickly, and in the other method it is required to provide two tubes.